Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of Materials Chemistry C
Paper
values for photocurrent generation efficiency and exciton
diffusion length.29,30 In this phase, the molecules are arranged
in a herringbone motif, with an efficient p–p stacking along the
b axis direction; the largest mobility values have been measured
along the b axis, so that conduction can be considered to orig-
inate from the efficient overlap between molecular p orbitals of
neighbouring molecules along the p–p stacking, as conrmed
by pressure dependence of charge carrier transport31,32 and also
demonstrated for similar derivatives.33 Also the perfect align-
ment along the short molecular axis between adjacent rubrene
molecules plays a key role34,35 in the triclinic polymorph of
rubrene,28 a small displacement of the molecules along the
short axis results in a poor charge carrier mobility.36
Scheme 1 Synthesis of propargyl alcohols.
The peripheral phenyl–phenyl repulsion, which results in a
strain in the tetracene core, is one of the factors at the basis of
the observed rubrene tendency to react with oxygen. In prin-
ciple, two approaches can be followed in order to reduce such
repulsion: the introduction of electron withdrawing substitu-
ents on two of these phenyl rings, which induces their efficient
p–p stacking, or the reduction of the steric hindrance of the
side groups by replacing phenyl rings with the smaller thienyl
rings.
Here, following both approaches, we describe the synthesis
of new rubrene derivatives and characterize them, focusing at
rst on the effects of substituents on the stability to photo-
oxidation of the different molecules in solution and on their
relationship with the oxidation potential and HOMO energy
level. Then, single crystals of all the new compounds are grown
and their structural characterization addressed, getting full
demonstration of a favourable crystal packing, close to that of
orthorhombic rubrene. Finally, charge transport measurements
are performed on all single crystals: the results fully assess one
of the new rubrene derivatives as an organic semiconductor
joining transport properties close to those of rubrene with a
much higher stability to oxidation. This result denitely
supports an oxygen-related process for charge transport
enhancement in rubrene, as opposite to a direct role of the
product of rubrene oxidation.
alternatively prepared by addition of an appropriate organo-
metallic reagent (e.g. a Grignard reagent of an arylacetylide or a
lithium acetylide) to diarylketones (such as benzophenone)46 or
by Sonogashira reaction between a 1,1-diaryl-propargyl alcohol
and a halogenated aromatic compound, according to a copper-
free protocol already applied to the synthesis of 1,1,3-triaryl
propargyl alcohols.39 A series of derivatives bearing some elec-
tron-withdrawing substituents (1a–c) was prepared in satisfac-
tory good yields following the Sonogashira protocol. In order to
compare these derivatives with another one bearing an electron-
donating moiety, we also prepared a propargyl alcohol (1d)
bearing a thiophene ring. Alcohol 1d was prepared following
both the above protocols.
For the conversion of these alcohols into rubrene derivatives,
we applied the one-pot protocol already described in the liter-
ature.39 Propargyl alcohols 1a–c were rst reacted in 1,1,2,2-
tetrachloroethane with mesyl chloride, in the presence of tri-
ethylamine, to transform the alcohol into the corresponding
chloro-allene. Then, the chloro-allene solutions were heated to
reux in order to convert these intermediates into the desired
tetraaryltetracenes 2a–c (Scheme 2). In the case of an alcohol
bearing a thienyl (1d), we developed a new protocol aimed at
Results and discussion
Synthesis
The interest about rubrene promoted a huge amount of effort to
synthesize, functionalize, and characterize derivatives of
rubrene.22–24,33,37–42 The most straightforward synthesis is based
on simple heating of 1,1,3-triaryl-3-chloro-allene (1,1,3-triaryl-3-
chloro-propan-1,2-diene), as described for the rst time in the
1920s and rationalized in the 1970s by Rigaudy.43 This method
is still used for the synthesis of commercial rubrene and has
been recently applied to the synthesis of some new derivatives
of rubrene.39,44,45 Due to the reduced number of synthetic steps
required by this approach and its compatibility with a wide
range of substituents, we selected it to prepare new derivatives
of rubrene, bearing electron-withdrawing substituents.
This rst step in the synthesis of rubrene according to the
allene protocol is the synthesis of appropriate triaryl-propargyl
alcohols (Scheme 1). Triaryl-propargyl alcohols can be Scheme 2 Synthesis of 5,6,11,12-tetraaryltetracenes.
4148 | J. Mater. Chem. C, 2014, 2, 4147–4155
This journal is © The Royal Society of Chemistry 2014